1. Field of the Invention
The present invention relates to the field of packaging flip-chip semiconductor device, and more particularly, to a flip-chip semiconductor device having a substrate covered with solder mask and a hole defined in the center of the substrate such that the process to under fill the semiconductor device is effective and the packaged semiconductor devices are reliable.
2. Description of Related Art
Flip-chip semiconductor devices are more and more popular because flip-chip technology permits a high component density and fast accessing time in a semiconductor system. FIG. 5 shows the structure of a conventional flip-chip semiconductor device, wherein a die (51) is attached to a substrate (53) in such a manner that the solder bumps (511) formed on the bottom face of the die (51) are combined with the solder pad (531) on the upper face of the substrate (53). One problem in such a flip-chip structure is that the coefficient of thermal expansion (CTE) of the die (51) is usually quite mismatched with that of the substrate (53). Therefore, the solder connection between the die (51) and the substrate (53) may be damaged when the operational temperature of the semiconductor device is too high and the die (51) and the substrate (53) expand in different degrees. To overcome such a thermal mismatch problem, encapsulation material (52) has to be filled into the space between the die (51) and the substrate (53) to encapsulate the solder bumps (511) thereby securely combining the die (51) and the substrate (53) together and decreasing the stress asserted on the solder connection between the die (51) and the substrate (53).
Further, to include the encapsulation material between the die (51) and the substrate (53), a dispensing needle (50) is used to dispense encapsulation material (52) along one side or two adjacent sides of the die (51) so that the encapsulation material (52) is then allowed to freely flow, as a result of capillary forces, beneath the die (51) and existing on the remaining sides. Such a dispensing process is time-consuming because encapsulation material (52) is dispensed along only one side or two sides of the die (51). Therefore, the manufacturing performance for the flip-chip semiconductor devices is difficult to increase. Moreover, it is likely to have voids generated in the encapsulation material (52) beneath the center portion of the die (51). Accordingly, when the packaged flip-chip semiconductor device is under SMD (Surface Mount Device) reflow, the air inside the voids is heated and expanded, causing the packaged semiconductor device to explode (known as popcorn phenomenon) or the die to crack. In addition, after the dispensing process, the fillet (55) formed on the dispensed side of the die (51) is larger than the fillet (56) formed on the side that is not dispensed. Therefore, it is not easy to better utilize space for the semiconductor device and the solder bumps (511) adjacent to the smaller fillet (55) are easier to be affected by thermal stress.
U.S. Pat. No. 5,710,071 granted to Beddingfielf et al. has provided an improved process for underfilling a flip-chip semiconductor device. With reference to FIG. 6, the process uses a substrate (63) with a hole (69) defined in the center thereof so that encapsulation material (62) can be dispensed along four sides of the die (61) at the same time and the air between the die (61) and the substrate (63) can escape through the hole (69) to avoid generating voids. Such a process is able to reduce the time required for underfilling because encapsulation material (62) is dispensed along all sides of the die (61). However, if the die (61) is not of square shape or the solder bumps (611) are not arranged in a symmetric form, as shown in FIG. 7, the encapsulation material (62) dispensed from four sides will flow asymmetrically due to the block of the solder bumps (611). That is, the encapsulation material (62) has a fast flow speed (V1) when it flows through an area with small amount of solder bumps (611). On the contrary, the encapsulation material (62) has a slow flow speed (V2) when it flows through an area with large amount of solder bumps (611). Therefore, the encapsulation material (62) with fast flow speed (V1) may reach and block the hole (69) while the encapsulation material (62) with slow flow speed (V2) is still flowing toward the hole (69). As a result, air is trapped between the die (61) and the substrate (63), and the dispensing process is not effective. Accordingly, there is a desired to have a process for under filling a flip-chip semiconductor device, which can eliminates the aforementioned problems.